Foamed thermoplastic products are currently made using either physical or chemical blowing agents. Physical blowing agents are gases or liquids at temperatures below the processing temperatures of the thermoplastics. Generally, with physical blowing agents it is difficult to obtain uniform cell distribution due to the lack of uniformity of application of the physical blowing agent because of the difficulty in mixing the physical blowing agent with the highly viscous polymer melt. Furthermore, the addition of large amounts of physical blowing agents which are dissolved into a polymer dramatically decreases the polymer glass transition temperature making a highly plasticized mixture which is processed at lower temperatures thus allowing the processor to maintain the thermal stability of the polymer. Physical blowing agents are generally used in the formation of low density foams which generally have a specific gravity of less than 0.1.
Chemical blowing agents decompose or interact upon being heated to a temperature below the processing temperature of the thermoplastic and liberate a gaseous product in order to expand the thermoplastic. Chemical blowing agents by themselves are not generally used in low density foams because they are expensive and produce a limited reduction in density by themselves.
Foams made from chlorinated polyvinyl chloride, in particular, retain many of the properties of chlorinated polyvinyl chloride polymers such as heat resistance, chemical resistance, and weathering resistance, as compared to other thermoplastic polymers. Therefore, the foam of a chlorinated polyvinyl chloride can be used in a wide variety of applications.
For example, Adachi, et. al., in U.S. Pat. No. 4,165,415 discloses a method for preparing both low and medium density foams of CPVC. The method comprises impregnating a chlorinated polymer of vinyl chloride with a foaming agent consisting essentially of a lower aliphatic monohydric alcohol having between 1 to 5 carbon atoms. The foaming agent is considered a physical blowing agent. The mixture is then heated to a temperature and for a time sufficient to cause the mixture to foam in a closed mold.
U.S. Pat. No. 4,772,637 to Kimura, et. al., describes the method of preparing pre-expanded particles of CPVC containing a large amount of inorganic materials. The CPVC, inorganic materials and a solvent are kneaded to form a gel. The kneaded mixture is pelletized and a physical blowing agent is impregnated into the pellets. The pellets are then pre-expanded. The pre-expanded particles are then placed in a mold and heated with a heating source to expand and fill the mold in order to obtain a foamed article.
The abstract of Japanese Patent No. 51024667 describes a heat and flame resistant CPVC resin having a chlorine content of 63-69% in a nitrobenzene solution. An organic foaming agent such as azodicarbonamide, or dinitrosopentamethylene tetramine along with processing agents such as heat stabilizers, and lubricants were added. These ingredients were then foamed.
The abstract of Japanese Patent No. 5148381 discloses a composition which is foamed to give a five (5) millimeter cellular sheet with a density (sic) of 0.74. The composition is obtained by compounding dibutyl tin maleate, dioctyl tin maleate, a chemical foaming agent and an acrylic processing aid with a vinyl chloride polymer. The chemical foaming agent can be an azo compound, an azide compound, a nitroso compound, and/or a sulpho-hydrazide compound. The acrylic processing aid is a homopolymer or copolymer of methyl methacrylate, ethyl methacrylate, butyl methacrylate, copolymers of alkyarylates. and so forth.
German Patent Disclosure Publication No. DE-OS 2302521 describes a process for the preparation of a flexible thermoplastic foamed material. The patent describes heating a mixture of chlorinated polyethylene and chlorinated polyvinyl chloride and a blowing agent in a closed space at a temperature above the softening point of the chlorinated polymer and above the decomposition temperature of the blowing agent. The blowing agent that can be used in the process is azodicarbonamide.
U.S. Pat. No. 4,383,048 to Hall, et.al., discloses the process of making low density chlorinated polyvinyl chloride foam having a density of 0.32 grams/cubic centimeter or less. The low density foam that is produced that has a substantially closed cell structure, low thermal conductivity and excellent thermal stability without the substantial amounts of stabilizer required when azodicarbonamide is used as the nucleating agent. The foam is produced using a primary blowing agent which could be gaseous nitrogen and an alkali metal borohydride in conjunction with a proton donor activator as the nucleating agent.
A chlorinated polyvinyl chloride composition to be foamed is described in U.S. Pat. No. 4,370,286. The composition comprises a chlorinated polyvinyl chloride resin containing at least sixty (60%) percent chlorine, an effective amount of a blowing agent, an effective amount of a nucleating agent, a processing aid selected from copolymers of styrene and unsaturated nitrile containing more than fifty percent (50%) of styrene and ten to forty percent (10-40%) of nitrile. The foam formed from the composition is a low density foam.
U.S. patent application Ser. No. 08/580,563, filed Dec. 29, 1995, discloses a medium density chlorinated polyvinyl chloride foam composition comprising a chlorinated polyvinyl chloride polymer containing at least sixty percent (60%) chlorine by weight, a nitrogen containing decomposition type blowing agent, a tin stabilizer, a costabilizer and a high molecular weight process aid. However, this medium density foam lacks good impact resistance properties. Comparatively, the present invention has improved impact properties or at a minimum displays a less brittle mode of failure from those compositions set forth in U.S. patent application Ser. No. 08/580,563. Furthermore, one of the components of blend of the blowing agents used in this application acts as a costabilizer for the CPVC itself, precluding the need for a separate costabilizer.
Thus, there currently exists a need for a composition to form a medium density chlorinated polyvinyl chloride foam in which such foam has good dynamic thermal stability and color stability and improved impact resistance. In particular, a need exists for a medium density CPVC foam having chemical resistance, good weathering characteristics, and high service temperature or Vicat softening temperature.